A vessel has been laid up for a considerable time with shore power connected. Routine underwater hull inspections reveal an unusually high deterioration rate of the vessel’s anodes. Assuming the dockside wiring, shore power connections and bonding systems are all in good condition and correctly connected:
(a) explain how this may occur;(8)
(b) state TWO devices that should be fitted to prevent this situation occurring.(2)
Accelerated Anode Deterioration in a Laid-Up Vessel
The accelerated deterioration of anodes on a vessel laid up with shore power connected is likely due to galvanic corrosion induced by stray currents.
Understanding the Issue:
- Stray Currents: Even with properly installed bonding systems and shore power connections, stray currents can originate from various sources within the marina or shipyard, such as ground faults in shore power systems, other vessels, or underwater cables.
- Anode Sacrifice: These stray currents can increase the electrical potential difference between the vessel’s hull and the surrounding water, causing the sacrificial anodes to corrode at an accelerated rate to protect the hull.
- Increased Current Demand: The extended connection to shore power might result in increased electrical loads on the vessel, leading to higher current demands and consequently, accelerated anode consumption.
Additional Factors:
- Water Quality: The quality of the water where the vessel is laid up can influence the corrosion rate. Waters with high salinity or specific contaminants can accelerate anode deterioration.
- Biofouling: The accumulation of marine organisms on the hull can create localized electrochemical cells, increasing corrosion rates and anode consumption.
To address this issue, it is essential to investigate the marina’s electrical system, conduct thorough inspections of the vessel’s electrical and bonding systems, and consider additional corrosion protection measures.
Two Devices to Prevent Accelerated Anode Deterioration
To prevent the accelerated deterioration of anodes on a laid-up vessel due to stray currents, the following devices can be fitted:
- Isolation Transformer: This device completely isolates the vessel’s electrical system from the shore power supply, preventing the flow of stray currents.
- Galvanic Isolator: While less effective than an isolation transformer, a galvanic isolator can reduce the flow of stray currents by acting as a barrier between the vessel and the shore power.
These devices help to protect the vessel’s electrical system and reduce the risk of accelerated anode corrosion.